Here's why:
* The "e" at the end of the word is silent. This means it doesn't affect the vowel sound.
* The "ea" combination in "leader" doesn't create the long "e" sound like it does in "tea" or "see". Instead, it's pronounced like the short "e" sound in "bed" or "pen".
So, "leader" has a short vowel sound, similar to the "e" in "bed".